1. Field of the Invention
The present invention relates to a sensor for measuring a desired variable.
2. Description of the Related Art
To cope with the strict demands on safety in the handling of complex machinery, the installation of a component of the machinery may be redundant or diverse.
In the case of a sensor, since the sensor under certain circumstances takes up relevant measured variables for the safe operation of the machine, redundant sensors may be installed for a proper working of the controller of a machine, particularly in the data transfer from the sensor to the processor. Redundancy may also be necessary in this area to guarantee the demanded safety conditions. A possible malfunction of individual components of the total measurement chain can be detected on the basis of the redundantly present measured results.
An exemplary realization of such a sensor-based measurement system of a machine whose sensor arrangement is of redundant design is shown in FIGS. 1a to 1c. The sensor-based measurement system has a conduit 1 conducting any desired medium, wherein the pressure of the medium present in the conduit 1 is detected by the sensor. The two measurement connections 2, 12, which are arranged mutually spaced apart along the conduit 1, serve this purpose. The medium within the conduit 1 to be checked is fed via the measurement connections 2 and 12 to the two measurement devices 3 and 13 which are respectively connected via the signal line 4 and 14 to a respective own processor unit 6 and 16.
As soon as a pressure pulse having an actual value in accordance with FIG. 1b occurs at the input of the conduit 1, it is continued in the conduit 1. Since the two measurement connections 2, 12 are mutually spaced apart, the pressure pulse reaches the measurement device 3 via the measurement connection 2 at the time t2 (see FIG. 1b) and reaches the measurement device 13 via the measurement connection 12 at the time t3.
Both the measurement devices 3 and 13 work independently of one another since no uniform clocking is provided. The measurement devices 3, 13 which can be seen from FIG. 1c form the analog electrical signals representing the actual value. The signals are transmitted via the two signal lines 4 and 14 to the two processor units 6 and 16. The two processor units 6 and 16 work independently of one another and thus not synchronously. The processors 6, 16 thus determine the measured values MW1 and MW2 corresponding to the actual value with a time offset with reference to the electrical values provided via the lines 4, 14.
To detect possible faults of the two independent measurement apparatus, a measured value comparison 50 is initiated between the two processor units 6 and 16. A substantial problem of the conventional sensor arrangement is caused by that the individual measured values MW1 and MW2 generated in dependence on time on the basis of the time difference 51 result in considerable measurement uncertainties. These measurement uncertainties must be observed in the comparison 50 of the measured results in that the region for the deviations from one another of the two measured values MW1 and MW2 to be tolerated is made larger. A malfunction of the conventional measurement apparatus is only diagnosed after an exceeding of the deviation tolerances.
For the foregoing reasons, there is a need for a measurement arrangement which provides greater security and a finer reaction possibility to possible measurement deviations.